The use of watchdog circuits to monitor the operation of microprocessors and to reset them in the event of aberrant operation is well known. Such circuits provide the reset command necessary to ensure that a microprocessor returns to a known reference point after a failure in order to resume a designated manner of operation.
Failure of a microprocessor to perform correctly during normal operation is typically a result of, though not limited to, any of three situations. The first is external electromagnetic interference (EMI), which can cause the power being supplied to a microprocessor to fluctuate, whereby the microprocessor produces unpredictable results. Secondly, external radiation, such as that found in outer space, can cause single event upsets (SEU) that also result in erratic operations. Finally, errors may also follow from unanticipated software bugs. These types of situations may cause erratic operation to occur at random intervals or continuously for extended periods of time. Thus, there is a need to reset microprocessors not only when a discrete interruption occurs but also to reset them continuously in the event of a prolonged interruption.
Furthermore, such resets often must occur quickly after a microprocessor enters an errant state of operation to prevent potentially hazardous or even fatal consequences, such as with use in an alarm system. Additionally, when power is supplied to microprocessors they are in undefined states of operation. Thus, an initialization or power-on reset is also necessary to ensure that a microprocessor will begin its designated mode of operation at the outset.
Accordingly, it is desirable to have a watchdog timer with the following features: a continuous reset for repeatedly resetting the microprocessor as necessary to return it to a normal state of operation; a quick response window to ensure that the microprocessor is reset quickly; and a power-on reset (POR) to reset the microprocessor following the initial application of electrical power.
The prior art does not teach how to combine these features within a single watchdog timer circuit. For instance, Younger (U.S. Pat. No. 5,341,497) discloses a circuit for detecting an anomalous operating state of a microprocessor that continually resets the microprocessor until a specified number of resets occurs, at which point an alarm is tripped. Younger further provides a limited response window feature for generating a reset upon occurrence of an errant state. However, Younger does not teach a method for implementing a POR feature. In like manner, Huang et al. (U.S. Pat. No. 4,627,060) discloses a watchdog timer with a limited response window feature and a POR feature, but no continuous reset feature.
In addition to the failure of the prior art watchdog timers to combine all of the above features in a single device, the current state of the art also does not allow the disabling of any of these features once they are in place. Current watchdog circuits constantly maintain control over their microprocessor reset inputs whenever power is supplied to the system. Yet, there are discretionary microprocessor functions which also must have access to the microprocessor reset input if they are to be performed, such as the ability to re-program microprocessors to perform alternate tasks.
As a result of this limitation, re-programming is typically conducted by physically removing the microprocessor from its circuit board and associated watchdog circuitry and placing it in a commercially available programming device. The programming device can then access the microprocessor reset without interference from the watchdog circuitry.
However, off-board programming can be problematic as well as time and labor intensive for numerous reasons. Microprocessors are typically the largest of all integrated circuits (IC) and accordingly have the most input and output (I/O) pins of all ICs. The delicate nature of the I/O pins requires meticulous handling to avoid damage thereof. Also, microprocessors are fastened to a circuit board by placing the I/O pins through tiny holes in the board and soldering the pins in place. Here again, the delicate I/O pins can be damaged from this fastening and unfastening.
The microprocessor, circuit board, and other circuit components can also be damaged during fastening and unfastening by the heat source required for soldering. Accordingly, the solder must be applied and removed from these pins with a narrowly focused heat source. Since microprocessors may have several hundred I/O pins, it becomes cumbersome to carefully remove the soldering from each of these pins. Additionally, ICs are susceptible to electrostatic discharge and can be damaged as a result thereof. Thus, it is desirable to handle these chips as little as possible so as to reduce the chances of such thermal and electrostatic damage.
Accordingly, there is a need for a watchdog timer that can be disengaged from microprocessor reset inputs, thereby allowing remote access to a microprocessor for in situ programming thereof.